I don't know about the apps, but using the basal body temperature method to track fertility is much older than the apps! It is not an exact science and should ideally be combined with another method of tracking fertility. The technology will only be as good as the data you enter.

You might want to consult with a doctor proficient in natural family planning methods to help you make best use of the technology.

This is what i was referring to, the basel thermometer readings. Since my post i have done more research and there is an app/service <moderator edit to remove name>where the app learns more about your body the more temperature readings you put into it and gives you more precise good/bad days.

They have just received $30,000,000 in funding, so hopefully the tech side of it can really take off.

As for any use of technology, the more accurate your data, the better the app might be at predicting what could happen in nature. Nature, however, does not care what our technology says. I'd consult a doctor before consulting an app. They've invested far more into their education.